The present invention is directed to automatic circuit testers and in particular to an arrangement for interconnecting test instruments to the tester's system pins.
A typical automatic circuit tester typically employs a "bed of nails" fixture, which includes a large number of spring-loaded probes ("nails") that contact test points on the device under test ("DUT"), typically a circuit board, at a large number of test points. These nails are in turn connected to instruments that drive or sense signals at the test points.
Although complete testing of a particular circuit board may require that connections be made between test instruments and a large number of test points on the board, the number of test instruments required in the automatic test equipment typically is only a small fraction of the total number of possible board test points, since only a fraction of the test points are typically in use in any given part of the test. The test instruments can therefore be multiplexed.
In an automatic circuit tester, the apparatus for performing this multiplexing and related switching is referred to as a scanner. The scanner is typically embodied in circuit boards that plug into the fixture along one edge and receive on their other edges signals that operate the scanner switches.
Historically, the circuit boards that carry the scanner switches have also included the instruments that the scanners couple to the DUT. This approach has operational advantages, since the instruments and their associated scanners should be kept as close to each other as possible. More recently, however, the marketplace has exhibited some resistance to this approach because it "locks the buyer in" to the use of instruments that the tester manufacturer supplies. The automatic-test-equipment industry has accordingly responded by moving toward "open architecture," in which different test-equipment manufacturers implement a common instrument-control protocol. They provide sockets that can be used by instruments manufactured by other suppliers so long as those instruments conform to the common protocol.
The instrument boards therefore do not include the (system specific) scanners. As a result, the user bears the burden of making the proper connections between the instrument and the various test points. He usually does this by providing multiplexer boards, typically also plugged into sockets that operate in accordance with the common control protocol, and wiring leads from the "front-panel" edge of the instrument board to the multiplexer board, which selectively connects the instrument input or output port to desired test points. The user can thus employ the same instrument for different test points at different parts of a test.
We have recognized, however, that the theoretical resultant flexibility yields practical inconvenience. The number of instruments and test points to which they need to be directed does not have to be very large before the necessary wiring between instruments and multiplexers becomes unwieldy. And even with the multiplexing, rewiring often proves to be necessary when tests are changed.